A Stirling cryogenic refrigeration system includes a reciprocating compressor which provides a sinusoidal variation in pressure in a working volume of gas. The working volume includes the head space above the compressor piston and the internal voids in a refrigerator cold finger. In the case of a split Stirling system, the working volume also includes a gas line interconnecting the above spaces. Within the cold finger, a displacer moves in timed relationship with the sinusoidal pressure variations to provide cooling at one end of the cold finger.
The usual Stirling cycle includes an electric motor driven compressor, and the invention will be described with reference to a split Stirling cycle of that type. The invention is also applicable to Gifford-McMahon type refrigerators such as disclosed in U.S. Pat. Nos. 2,906,101 to McMahon et al. and 2,966,035 to Gifford in which pressure control valves are positioned between the compressor and the cold finger.
Generally, the fluid seal about the compressor piston is an annular lip or split ring seal. One problem encountered in Stirling systems results from the fact that there is always some leakage past the dynamic seal between the reciprocating compressor piston and its cylinder. The leakage is between the working volume and a control volume on the opposite side of the piston. Where there is a greater leakage in one direction than in the other with movement of the piston, the mean value of the pressure in the working volume of gas tends to be above or below the mean pressure of the control volume. If the direction or magnitude of that leakage changes during the life of the refrigerator, the refrigerator suffers a change in the pressure wave for which the cold finger and displacer were designed. The result is a significant drop in the efficiency and capacity of the device.
An object of this invention is to provide a Stirling refrigeration system, and a compressor for use therein, wherein the mean value and the peak to peak differential of the sinusoidal pressure variations in the compressed working volume of gas is stabilized for a long refrigerator life.
A problem common to both Stirling and Gifford-McMahon refrigerators is that, with wear, particles from the compressor seals contaminate the helium refrigerant. Those contaminants result in a significant degradation of performance and shorten the operating life of the refrigerator.
A further object of this invention is to provide a refrigeration system having a longer life than prior systems due to lesser wear to the compressor section of the refrigerator.